It pays to mix it up—planted forests containing more than one tree species can store several times as much carbon as monocultures, as shown in a meta-analysis published in Frontiers in Forests and Global Change.
Researchers have long known that biodiversity increases forest productivity, but the effects demonstrated in the recent publication are “quite striking in terms of the magnitudes,” said forest management and ecology expert Kris Verheyen from Ghent University, who is not an author on the study but has worked with the authors.
Forestry companies often plant monocultures, so the study has the potential to affect industry practices. The connection to real-world applications is “what I found quite cool in the article,” said ecologist and meteorologist Rémy Beugnon from the German Centre for Integrative Biodiversity Research.
The Value of Variety
Researchers sifted through more than 11,300 studies, including some from a worldwide network of tree diversity experiments called TreeDivNet, to find 18 that included the information necessary to compare carbon storage in monocultures with that in stands containing two or more species of trees.
Initial results indicate that monocultures couldn’t keep up with mixed forests, the researchers found. Stands with two or more species contained at least 25% more aboveground carbon than the best-performing monocultures, although the researchers note that more data are needed on this comparison. When the researchers focused on forests containing four species, the effect became more striking. These mixed stands had more than 4 times as much carbon as the average monoculture and more than twice as much carbon as the best-performing monocultures.
How many tree species a forest needs to maximize its productivity is an open question, said ecologist Emily Warner from the University of Oxford, lead author on the study. She and her colleagues found that carbon storage peaked with four species, but the studies they analyzed were skewed toward young forests containing few species, which may have influenced the result.
“It’s not just about planting trees; it’s about planting diverse forests.”
Many countries and companies are using tree planting to offset carbon emissions. But these results show that “it’s not just about planting trees; it’s about planting diverse forests,” Warner said.
There are multiple reasons why mixed stands might be more productive than monocultures, Warner said. Different species may have complementary ways of using resources, for example, so mixtures of these species may coexist more easily than stands containing only one species.
Trees with the ability to fix nitrogen—meaning they turn nitrogen from the air into a form usable by other plants—did not increase forest productivity any more than a non-nitrogen-fixing species, the researchers found. This result contrasts with what’s been seen in grasslands, and it could relate to the young ages of the forests studied, Verheyen said. “It takes a while before the nitrogen starts to accumulate, really, in the system, and maybe this is something that will play out also in the longer run,” he added.
“It’s a great article,” Beugnon said, and there’s room for future studies to add to it. In particular, he called out the fact that none of the studies the authors considered were based in South America or Africa—the two continents where most reforestation is occurring.
Future studies could also consider the carbon stored belowground. It’s tough to find such data because analyzing belowground carbon requires digging up tree roots. That’s something foresters are reluctant to do when they’re trying to establish a plantation, said ecologist Catherine Potvin from McGill University, an author on the study.
Logistical Challenges
Logistical challenges inhibit forestry companies from planting diverse forests, Potvin said. Machines used to plant trees tend to be most appropriate for single species, and harvesting multiple species is more complicated than harvesting a monoculture. Different species may have different growth rates, leading to different trunk sizes or branch heights that require different harvesting practices, for example. Forests also take many years to grow, so foresters tend to be averse to trying new species or techniques because they won’t know the value of their investment for decades.
Monocultures, traditionally seen as low-risk forestry operations, “might in fact become a risky forestry operation given climate change.”
Forestry companies are among the few groups with the capacity to plant acres upon acres of trees, so the restrictions they face have widespread impacts. Even plantations intended for uses other than wood production—such as the carbon market—are usually planted by these companies, Potvin said.
With global temperatures rising, foresters may find themselves more motivated to try new techniques, however. Trees are now subject to new stresses, and “what was a nonrisky forestry operation might in fact become a risky forestry operation given climate change,” Potvin said. In addition to making forests more productive overall, investing in multiple species could provide a type of insurance against one species reacting badly to new weather patterns.
Fortunately, even adding a small amount of biodiversity can be beneficial. When it comes to increasing forest productivity, Warner said the results of the meta-analysis show that “even including just one more species is worth doing.”
—Saima May Sidik (@saimamaysidik), Science Writer